Referring to FIGS. 1 and 2, a conventional lifting device 1 includes an outer tube 11, an inner tube 12 movably inserted into the outer tube 11, and a support module 13 disposed between the outer and inner tubes 11, 12. The support module 13 includes three spaced-apart columns of ball units 14, each column of which has a plurality of balls 141. The balls 141 are sandwiched between the outer and inner tubes 11, 12. When the inner tube 12 is moved relative to the outer tube 11 along an axial direction, the balls 141 will rotate to permit smooth movement of the inner tube 12 relative to the outer tube 11.
Since the sizes of the balls 141 depend on a gap between the outer and inner tubes 11, 12, when there is a slight change in the diameter of the inner tube due to production error, the gap between the outer and inner tubes 11, 12 will change. As a result, the movement of the inner tube 12 relative to the outer tube 11 is unstable. If the gap between the outer and inner tubes 11, 12 is excessively large, each ball 141 will have to be replaced with a larger diameter. Hence, the support module 13 of the conventional lifting device 1 cannot provide a stable support due to the changes in the gap between the outer and inner tubes 11, 12.